1. Field of the Invention
The present invention relates to an annular extrusion die for extruding tubular thermoplastic materials, such as foamed thermoplastic, and more particularly, relates to an annular extrusion die incorporating an internal spider mandrel and a novel choke ring for eliminating striations in the extruded thermoplastic material while imparting an improved control over the uniformity in the gauge or thickness of the extruded plastic material.
Although the technology of extruding tubular thermoplastic materials, such as foamed plastics which are subsequently employed in sheet form for the thermoforming of various articles; for example, plastic trays, cups, egg cartons or the like, is presently well developed and advanced, various problems are still in evidence in the extrusion of foamed plastics possessing a satisfactorily constant gauge thickness across its expanse and, moreover, which is devoid of weld lines or striations produced by the spiders supporting the internal mandrel of the annular extrusion die. Within the annular extrusion die, the flow of the thermoplastic material melt or polymer from the extruder to the extrusion die nozzle or orifice is subjected to separations in regions of the flow passageway radially traversed by spiders or webs through which an internal mandrel is supported within the body of the extrusion die, and with the separate flows of the melt then rejoining within the flow passageway downstream of the spiders supporting the mandrel. Generally, as the separated flows of the thermoplastic melt rejoin, at the junctures of the flows there are produced weld lines or striations which, subsequent to the extrusion of the tubular thermoplastic material, especially when formed into foamed plastic sheeting, represent weakening lines in the thermoplastic material tending to adversely affect the physical strength and flexibility of any articles subsequently thermoformed therefrom.
In other annular extrusion dies, in lieu of the spiders supporting the internal mandrels within the die body so as to form the flow passageway for the thermoplastic melt, a ring-shaped support plate extends across the flow passageway, with a plurality of through apertures being formed in the plate to permit the flow therethrough of the plastic material in the shape of numerous individual strands which then rejoin within the annular extrusion die flow passageway downstream of the plate to form a single material stream. However, the use of such an apertured or perforated plate causes the formation of a large number of weld lines or striations within the material, generally commensurate with the number of plate apertures, which may be unacceptable in the production therefrom of the thermoformed articles. The use of spiders for supporting the internal die mandrel considerably reduces the number of weld lines or striations in the extruded thermoplastic material, but still causes some but still weld lines or striations to be produced in the extruded material in regions where the flows of the melt regions in the extrusion die downstream of the spiders.
In order to alleviate the problems encountered in the formation of striations or weld lines in the thermoplastic material which is caused by the flow separations occasioned by the spiders of the internal die mandrel, annular flow restrictors or choke rings have been positioned so as to extend into the flow passageway of the extrusion die downstream of the spiders structure, providing a narrowed gap in the flow passageway and generating a back pressure upstream of the narrowing forcing the flows of material to more intimately admix upon rejoining so as to extensively obturate or eliminate any weld lines or striations produced in the material by the spiders. Generally, this has been accomplished in the technology by regulating the size or width of the annular gap formed by the restrictor or choke ring in the flow passageway of the extrusion die. Notwithstanding the alleviation of the various technological problems caused by striations through the use of restrictors or choke rings, these have not proven completely satisfactory in providing an accurate control over gauge thickness of the entire surface of extruded foamed thermoplastic material.
Although as mentioned hereinabove, the insertion of a choke ring to produce a predetermined annular flow restriction and generating back pressure to obturate striations in the thermoplastic materials, has alleviated many of the problems encountered with respect to the physical proper ties and strength of the extruded foamed thermoplastic, difficulties have been encountered in accurately controlling the gauge of the extruded material within requisite bounds. This is caused by the mathematical relationship between the flow rate of the plastic material changing to the third magnitude (cubed) with regards to changes in the width of the gap between the choke ring and the facing wall of the flow passageway. Thereby, even a slight change or variation in the gap width produces considerable changes in the quantity of material being extruded and in the resultant gauge thickness, and may lead to overcontrolling or "overshooting" any desired changes in the gauge thickness of the extruded plastic foam material.
2. Discussion of the Prior Art
Although extensive consideration has been devoted in the technology to the elimination or alleviation of striations in the thermoplastic foam extrudates caused by spider mandrels, there is a need to provide a more accurate control over the gauge thickness of the extruded material.
Sokolow U.S. Pat. No. 3,899,276 discloses an annular extrusion die with back pressure control over a polymer melt through the positioning of a restriction-forming choke ring in the flow passageway of the extrusion die downstream of a spider arrangement for an internal mandrel. The choke ring is adapted to restrict the cross-section by a predetermined extent to generate a controllable back pressure alleviating the formation of weld lines or striations in the extruded thermoplastic polymer melt. Pursuant to a modified embodiment, the choke ring rather than being mounted on the die body so as to extend radially inwardly in the annular flow passageway for the polymer melt, is mounted on the internal mandrel so as to extend radially outwardly to the passageway.
Devine U.S. Pat. No. 1,768,671 discloses a tubing machine incorporating an annular extrusion die having a spider mandrel supported within the body of the die through the intermediary of a plurality of radially extending webs, and includes an annular restrictor or choke ring downstream of the spider web in order to generate a back pressure tending to heal or obliterate weld lines or striations formed in the polymer extrudate.
Ratheiser U.S. Pat. No. 4,045,154 discloses an annular extrusion die for forming tubular foamed plastics, in which an internal spider mandrel is supported through the intermediary of a plurality of circumferentially spaced spider webs having aerodynamic cross-sections to enhance the rejoining of the separated flows of the polymer melt being conducted through the extrusion die, and in which grooves are formed in the facing surfaces of the mandrel and extrusion die body forming the annular passageway to obturate striations or weakening weld lines in the polymer material.
Norris U.S. Pat. No. 1,182,199 discloses a machine for molding tubular structures from plastic compounds, including an extrusion die having a mandrel supported therein by spiders, and with a flow restrictor producing a pressure build-up in order to compensate for weld lines or weakened sections in the tubular material caused by the presence of the spiders supporting the internal mandrel.
Porter U.S. Pat. No. 2,987,766 discloses a tubular extrusion die incorporating a flow restrictor imparting a change in width to the cross-section of an annular flow passageway for the plastic material so as to compensate for differences in the gauge of the extruded material.
Finally, Egli, et al. U.S. Pat. No. 3,976,419 discloses a annular extrusion die including the restrictor proximate the die orifice for improving the surface gloss and reducing the interior surface roughness of foamed thermoplastic materials.
Although each of the above-mentioned publications alleviate to some extent the problems encountered in the formation of weld lines or striations extruded tubular thermoplastic materials, such as foamed thermoplastics, which are produced by spiders supporting internal mandrels, the utilization of currently known choke rings is considered inadequate controlling the uniformity of the gauge of the extruded material by regulation of the back pressure about the circumference of the flow passage of the extrusion die through compensation of pressure differentials occasioned by the spiders of the spider mandrel of the die.